1. Field of Invention
This invention relates to improved terrain mapping radar processing systems, and more particularly to a forward squint, coherent pulse doppler synthetic aperture mapping radar having full three dimensional, distortionless mapping capability.
2. Description of the Prior Art
Coherent pulse doppler synthetic aperture radars of the ground mapping type are well known in the art. As is known, the principle object of one form of such a radar is to derive high resolution maps over a large area of terrain by means of processing of the coherent return received from a reflecting target area illuminated with electromagnetic waves, such as by a radar transmitting antenna, where the illuminated area is much greater than the desired map resolution. In such processing radars, the returns received by the radar antenna are first separated in accordance with range, thereby providing a plurality of range strips which are at distinct radial distances from the illuminator, or radar. Such systems also separate signals in accordance with the doppler frequency of returns received, which frequency is the difference between the frequency of the transmitted wave and the frequency of the received wave resulting from compression or expansion of the reflected wave due to a closing or opening velocity of the antenna with respect to a given reflector. Naturally, a map is defined when each resolvable point of intensity is expressible by the analysis of signals which have in the information content thereof, designations of range and a second coordinate such as relative angle with respect to the flight path of the radar antenna. Thus, the synthetic aperture radar provides, through signal processing, extremely high resolution, equivalent to that achievable with an antenna of a much larger size.
Mapping radars of this type produce, basically, reflectivity maps. That is, the map is substantially a matrix of the intensity of reflections received from the area. However, reflectivity of most objects varies in dependence upon the angle at which they are illuminated. Additionally, since one use for a radar map is to compare it with later radar maps of the same area, thereby to determine position and other parameters through map matching, it is desirable that radar maps be reproducible to some comparable correspondence and quality. However, the reflectivity of objects in most cases varies as a function of time and weather, as well as being highly dependent upon the direction of incidence and direction of reflection.
As the radar-carrying aircraft flies along its path over the sampling distance (that distance between the time when a given target is first picked up and monitored and the last time at which the target is monitored), the target response scintillates, in dependence on its electromagnetic and geometric properties, and on the particular angle at which it is illuminated by the radar. Therefore, not only does the phase of the target response change over the sampling interval, but there can be significant differences in the amplitude of response over the sampling interval. When non-coherent map averaging is employed, the phasors (of which each response is comprised) may in fact average to zero over a long enough sampling interval; in such cases, therefore, only the average of the magnitude is utilized, and the phase information is discarded. This non-coherent integration comprises a form of non-coherent integration which has been used in prior art systems in an attempt to suppress scintillation. Because the magnitude of radar reflectivity is integrated on a cell-by-cell basis over the region being mapped during a mapping flight, there is a loss of map resolution and a loss of effective average power as a consequence of terrain scintillation. This mitigates against the conservation of bandwidth; and, conservation of bandwidth is, of course, desirable in a high quality system handling such a large amount of information.
Another desired quality of maps made by radar would be that they bear a resemblance to aerial photographs of the same area, or to base, or topological maps. However, except for highly distinguishing features (such as a harbor), radar maps obtainable heretofore of miscellaneous terrain bear inadequate quantitative resemblance to aerial photographs or topological maps. It therefore becomes difficult to interpret characteristics of the area from the map, or to recognize and accurately locate land marks of familiar terrain on a radar map.
The coherent pulse doppler synthetic aperture processing radar, as shown to the art, utilizes range gating and doppler filtering to allocate each return signal received at the radar to a range/doppler cell. In the case of a forward or backward squint synthetic aperture radar (in contrast with a strictly side-looking radar), the processing includes compensation in range and doppler for changes therein which result from the relative velocity of the antenna system with respect to a given reflector. Thus, as an aircraft closes on a target (although not necessarily flying directly toward the target), not only will the range decrease, but the doppler frequency, in most cases, will decrease, eventually becoming zero as the aircraft flies by the target broadside, at minimum range. Therefore, even though successive return signals from a given reflector or target have different range and doppler values, they can be allocated through processor compensation to the same target, and the complex phasors of the received signals resulting from that target can be integrated over a large number of signals. The actual resulting output for a given reflector is an electrical phasor allocated to a particular point on a two dimensional matrix representing actual positions on the terrain being mapped (or otherwise illuminated by the radar). Stated alternatively, a synthetic aperture mapping radar relates an arbitrary two dimensional coordinate matrix on the ground to the position and velocity of the antenna-carrying aircraft, and then allocates phasors to each of the cells in the two dimensional matrix. The phasors may be expressed in terms of a magnitude at a phase angle, or in terms of an in-phase component and a quadrature component.
In considering such processing radars, it is well to bear in mind the distinction between the frequency shift or doppler frequency which results from the relative velocity of the antenna with respect to the reflector, and the actual phase of the signal received, resulting from total time of travel of the electromagnetic energy to and from the terrain point, and the phase shift occurring upon reflection.
In the type of radar described, points at an equal distance, or slant range, from the reflector form a sphere having its center at the antenna; the sphere has a thickness in range due to the minimum resolvable unit of range. Points of equal doppler frequency fall on a cone having the velocity vector of the aircraft as its axis; the cone has a finite cross-range thickness due to the minimum resolvable unit of doppler frequency. The intersection of the range sphere with the doppler cone for any given pair of range and doppler comprises a range/doppler annulus.
One of the major problems of synthetic aperture mapping radars of the type known to the art is that, because of the absence of contrary data, any point on a range/doppler annulus is taken to be a point on an assumed plane of flat terrain. However, as is discussed more thoroughly and illustrated hereinafter, the actual position of the reflector cannot be resolved within the annulus. In a prior art processing radar, a reflector which is at a relatively low elevation, near the bottom of the annulus, and relatively closer to the ground track of the aircraft is inseparable in location from one which is at a higher elevation and relatively farther away from the ground track of the aircraft; both points being on the same range/doppler annulus. This causes two distinct problems: the first problem is that this results in significant errors in the computed position of a reflector; this is further compounded by the fact that it renders still more pronounced, the difference in configuration of reflectivity maps which are made from varying angles of approach to a mapped area. The amount of error which results from the range/doppler ambiguity increases with increasing grazing angle: that is, the errors are greater when the altitude of the aircraft is high with respect to the distance of the target area from the ground track of the aircraft, and the errors are lesser when the target area is at a great distance from the ground track of the aircraft with respect to the altitude of the aircraft. In an X, Y, Z coordinate system in which the X axis is the ground track of the vehicle, the Z axis is elevation, and the Y axis is in the ground plane perpendicular to the ground track of the vehicle, the X, Y plane at Z=ZERO is taken to be that plane which is parallel to the earth's surface and below the aircraft by a distance which is equal to the given altitude (H) of the aircraft, as determined by on-board navigational equipment. In other words, the mapped terrain is presumed to be a plane which is H feet below the aircraft, H being the nominal, navigational altitude of the aircraft. Deviations of given points in the area being mapped from this nominal plane (actual target elevation) result in cross track errors, as proven in equations (17) through (19) hereinafter, as follows: EQU .DELTA.Y=-H/Y .DELTA.Z (1)
As examples, consider an aircraft flying at 12,000 feet. For a target area which is 4000 feet off the ground track of the radar, a target height of 200 feet will make a 600 foot cross track error in the map; a target which is two nautical miles off the ground track of the radar-carrying vehicle (which distance is equal to the 12,000 ft. altitude of the aircraft), having an elevation of 200 feet would cause a cross track map error of 200 feet; for a map section which is three nautical miles off the ground track of the radar, the target at an elevation of 200 feet would be located with a cross track error on the map of about 133 feet.
It should also be obvious that a great difference in this error, and in the reflectivity map (which results from known pulse doppler synthetic aperture mapping radars), will occur in dependence upon the angle from which mapping of a region is accomplished. Thus, if an aircraft flew due south, while east of a region and mapped it, positive elevations would cause targets to appear on the map more easterly than they should be. Conversely, if the plane flew due north while westerly of the same region, the elevation would now cause the target to appear, erroneously, more westerly on the map than it should. Thus not only are reflections different from different viewing angles, but map errors resulting from the ambiguity of the range/doppler annulus cause significant differences in the location on the map of a given reflective area having an elevation above the nominal ground plane.
It should further be obvious that even though the average terrain being mapped may be at a given elevation with respect to an arbitrary reference plane, adjacent land marks of vastly differing elevation will appear highly separated on the map (when in fact they are adjacent) as a result of the range/doppler ambiguity just described. Further, a tall, narrow land mark will appear, instead, as a short, wide land mark as a result of the differential error of differential elements of elevation of the tall land mark, in a fashion described with respect to equation (1) hereinbefore.
Radar mapping is likely, in general, to be performed in a noise and clutter environment. As is known, summing of the radar metric over several maps should cause the undesirable components (noise and clutter) to average out, thereby increasing the effective signal power. However, reflectivity maps of the type known to the art are substantially incapable of corresponding multiple production; that is, no two maps of the same terrain are sufficiently alike to permit addition without desired map metric signals actually cancelling each other. Therefore, information is actually lost, rather than enhanced, by averaging of prior art maps.
Coherent pulse doppler radars are also used for moving target separation, identification and tracking in which the nature of the radar (the varying doppler frequency of returns received at the antenna) is used to distinguish between different relative velocities of targets with respect to the radar-carrying vehicle: the resolution of adjacent points of terrain is not involved. In such cases, the tracking of the target supplies location information to that target, and a relative elevation of the selected target may be determined by using the techniques of an interferometer radar or of a monopulse radar. The known interferometer radar utilizes relative electrical phase difference of return waves received at two antennas vertically separated from one another to determine a depression angle with respect to a plane perpendicular to the line joining the centers of the two antennas. The known monopulse radar utilizes either phase or amplitude resolution between a pair of radar beams. However, prior tracking radars are unable to resolve the range/doppler annulus ambiguity, and are incapable of generating a terrain map. The tracking radar is useful only when a selected, dominant target is uniquely identified, and uncorrected, general depression (or elevation) is provided only with respect to a single doppler cone.
Thus, there are available a wide number of radar processing techniques for the resolution of targets and the identification of positional information with respect to those targets. However, there is not heretofore available a system for providing high resolution radar maps which accurately describe the terrain, topology and landmarks of the area being mapped, nor which are reproducible to a useful degree of correspondence without regard to the relative velocity vector direction of the mapping vehicle when making successive maps, or electromagnetic response characteristics of the map area which may vary with time and aspect. Additionally, systems capable of producing high resolution synthetic aperture three dimensional maps of the three dimensional world have heretofore been unavailable.